The preparation of glucose from starch is a commercially important process, especially because of the subsequent use of glucose as a feedstock in its enzyme-catalyzed isomerization to fructose. The conventional scheme for glucose production is depicted in FIG. 1.
In FIG. 1 thinned starch, 1, which is a partially hydrolyzed starch more completely defined below, is the feedstock, at a dry solids (DS) level of about 30-40%, entering a saccharification or hydrolysis reactor zone, 2, where it undergoes enzyme-catalyzed hydrolysis using a glucose-forming enzyme, for example, amyloglucosidase (glucoamylase), hereafter referred to as AG. An essential feature of present processes is that hydrolysis is continued until maximum glucose formation is attained, which corresponds to about 94-96% glucose in the product stream, 3, when using a feedstock containing about 30% dry solids. Although only one reactor zone is depicted for saccharification, this is but one embodiment, and a plurality of reactor zones in series may be used in other embodiments.
Effluent, 3, from 2 containing more than about 94% glucose (on a dry solids basis) is then concentrated, where necessary, in an evaporation zone, 4, to afford a product stream, 5, containing from about 35% to about 50% dry solids. This product stream, 5, is typically the feedstock entering an isomerization reactor zone, 6, in which glucose is enzymatically converted to fructose by glucose isomerase.
The preparation of maltose from starch is also a commercially important process with a scheme analogous to that depicted in FIG. 1, the major difference being that the thinned starch is hydrolyzed with a maltose-producing enzyme. Beta-amylase is presently largely, if not exclusively, the maltose-producing enzyme used. Additionally, somewhat different alpha-amylases may be used to produce thinned starch with differing, but analogous properties depending upon whether maltose or glucose is desired.
The invention herein is directed toward both glucose and maltose production. However, solely for the sake of brevity the description herein will be directed toward glucose production, with it being clearly understood that a similar description directed toward maltose production is subsumed therein.
Several disadvantages attach to the aforedescribed process. One disadvantage, generic to any run to maximum conversion, is the increased cost consequent to the process time requirements for attaining maximum conversion; the longer the time for such conversion to be established, the more costly, hence more disadvantageous, is the process. Because glucose represses enzyme activity by complexing with AG, this leads to a decrease in hydrolysis rate as glucose accumulates and further increases process time. Another disadvantage, characteristic of the AG-catalyzed hydrolysis of thinned starch, is that the formed glucose, a monosaccharide, reverts to disaccharides, among which is isomaltose. Because isomaltose is a refractory disaccharide, that is, it is not readily hydrolyzed, and because it is bitter, it is a highly undesirable component of a glucose feedstock used for fructose production. Yet the longer the reaction time, the higher the glucose level, the higher is the isomaltose concentration in the product.
Because present commercial processes for production of high fructose corn syrup by isomerization of glucose utilize a glucose feedstock containing at least 94% glucose, a constraint of any new or modified process for production of glucose is that it afford comparable glucose levels. The process of this invention achieves such results by a method where hydrolysis proceeds to an extent short of maximum glucose formation to afford a product from which at least 94% glucose can be obtained by separation, with recycling of the stream from the separation stage depleted in glucose to the enzyme-catalyzed hydrolytic stage. By carrying out hydrolysis to a state substantially short of maximum glucose formation, the invention herein achieves a considerable saving in time and affords glucose with substantially lower levels of reversion products.
Therefore, an advantage of our invention is that it affords a substantial reduction in process time. Another advantage accompanying a reduction in time is that the process which is our invention affords glucose with less reversion products than the prior art processes.
Still other advantages accrue from characteristics of enzyme-catalyzed hydrolysis of thinned starch which the presently used commercial processes cannot take advantage of. When a feedstock for AG-catalyzed hydrolysis is increased in dry solids it is found that enzyme stability, as measured by its half-life, also increases. It is also found that the rate of glucose formation increases with increasing dry solids. Both of these characteristics are quite favorable, yet cannot be used in present commercial processes because increasing dry solids also leads to a lower maximum glucose level accompanied by increased reversion products.
In contrast to the prior art methods, the process of the instant invention is able to advantageously utilize the favorable characteristics of increased enzyme stability and increased glucose formation rate without any accompanying disadvantage of increased reversion products. Thus, in this sense our invention is truly synergistic; it incorporates the beneficial effects without incorporating the detrimental one.
The characteristic of using immobilized AG in hydrolyzing thinned starch is that it typically affords less than 94% glucose at equilibrium. Thus, immobilized AG can be used only with difficulty in present commercial processes. Therefore, yet another advantage of the instant invention is that it readily permits the use of an immobilized AG.
Still another advantage of the process described herein is that it affords a substantial increase in productivity, defined as the amount of glucose formed per unit of enzyme. This productivity increase results, in part, from recycling the enzyme incidental to the recycle stage of the process (where soluble AG is used), as well as a longer half-life (where either a soluble or immobilized AG is used).
The glucose level in our process as described is at least 94%. However, glucose levels of greater than 99% may be readily achieved by suitably varying process variables. Thus, still another advantage of our process is that it may be tailored to continually produce high-purity glucose, with a glucose purity greater than 99% being attainable.
Yet another advantage of the process which is our invention is that it can afford virtually complete conversion of starch to glucose.
It should be readily apparent from the multitude of the aforementioned advantages that our invention represents a substantial advance in the art of producing glucose at levels of about 94% and greater by AG-catalyzed hydrolysis of thinned starch.